Subject: Re: [PATCH] veth: Showing peer of veth type dev in ip link (kernel side)

From: [email-masked] (Eric W. Biederman)
Date: Tue, 15 Oct 2013 13:34:39 -0700

Nicolas Dichtel <[email-masked]> writes:

> Le 10/10/2013 02:17, Eric W. Biederman a écrit :
>>
>> Right.
>>
>> IFLA_NET_NS_PID is not invertible as there may be no processes running
>> in a pid namespace.
>>
>> IFLA_NET_NS_FD is in principle invertible.  We just need to add a file
>> descriptor to the callers fd table.  I don't see IFLA_NET_NS_FD being
>> invertible for broadcast messages, but for unicast it looks like a bit
>> of a pain but there are no fundamental problems.
> I'm not sure to understand why it is invertible only for unicast message.
> Or are you saying that it is invertible only for the netns where the
> caller stands (and then not for the veth peer)?

The pain is that it is a special case of SCM_RIGHTS aka passing file
descriptors.  Right now we don't support SCM_RIGHTS on netlink sockets
and so from that perspective IFLA_NET_NS_FD is a bit of a hack.

For unicast messages we can just stuff a file descriptor in the calling
process and be done with it.  For multicast messages we have to be much
more complete.

>> I don't know if we care enough yet to write the code for the
>> IFLA_NET_NS_FD attribute but it is doable.
> I care ;-)
> Has somebody already started to write a patch?

For IFLA_NET_NS_FD not that I know of.

Mostly it is doable but there are some silly cases.
- Do we need to actually implement SCM_RIGHTS to prevent people
  accepting file-descriptors unknowingly and hitting their file
  descriptor limits.

  In which case we need to call the attribute IFLA_NET_NS_SCM_FD
  so we knew it was just an index into the passed file descriptors.n

- Do we need an extra permission check to prevent keeping a network
  namespace alive longer than necessary?  Aka there are some permission
  checks opening and bind mounting /proc/<pid>/ns/net do we need
  a similar check.  Perhaps we would need to require CAP_NET_ADMIN over
  the target network namespace.

Beyond that it is just the logistics to open what is essentially
/proc/<pid>/ns/net and add it to the file descriptor table of the
requesting process.  Exactly which mount of proc we are going to
find the appropriate file to open I don't know.

It isn't likely to be lots of code but it is code that the necessary
infrastructure is not in place for, and a bunch of moderately hairy
corner cases to deal with.

Eric